The Internet and Packet Switching

Overview

Recall questions on the structure of the Internet, how packet switching works, its advantages and the role of a router.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Describe the structure of the Internet.

   Model answer

   The Internet is a global network of interconnected networks — a vast wide area network formed by joining together many smaller networks. Individual computers connect to a local network, local networks connect to an Internet Service Provider, and Internet Service Providers connect to one another and to high-capacity backbone links, often fibre-optic cable and undersea cable, that carry traffic between regions and continents. There is no single owner and no central computer through which everything passes: the networks agree to interconnect and to route each other's traffic using common protocols.

2. 2

   Explain packet switching.

   Model answer

   In packet switching a message is broken into small blocks called packets, each carrying the destination address as well as its share of the data. Each packet is sent independently across the network, and routers forward each one along whatever route is best at that moment, so different packets of the same message may travel by different routes and may arrive out of order. At the destination the packets are checked, any that are missing or corrupted are requested again, and the rest are reassembled into the original message using their sequence numbers.

3. 3

   Give two advantages of packet switching.

   Model answer

   Any two of: the links are shared efficiently, because many communications can be interleaved on the same line rather than one conversation tying up a whole circuit; it is resilient, because if a link or router fails the remaining packets can be routed around the problem; congestion can be avoided by routing packets round busy parts of the network; and a corrupted packet can be resent on its own rather than the whole message being resent.

4. 4

   What is the role of a router in packet switching?

   Model answer

   A router sits at a junction between networks and forwards packets on towards their destination. It reads the destination IP address in each arriving packet, consults its routing table to decide which of its links is the best next step towards that address, and sends the packet out on that link. It makes this decision independently for every packet, so it can respond to congestion and to failed links.

5. 5

   What does a packet consist of?

   Model answer

   A packet has three parts. The header, which carries the addressing and control information: the sender's IP address, the recipient's IP address, the protocol in use, the packet's sequence number so it can be reassembled in the right order, and a time to live or hop limit. The payload or body, which is the piece of the actual data being carried. And the trailer or footer, which marks the end of the packet and carries an error-checking value such as a checksum or cyclic redundancy check so the receiver can tell whether the packet arrived intact.

6. 6

   What is the purpose of the time to live field in a packet, and what happens when it runs out?

   Model answer

   It stops a misrouted packet circulating forever. The field holds a number that is decreased by one at every router the packet passes through. If it reaches zero the packet is discarded rather than forwarded, and the router may notify the sender. Without it, a routing error that formed a loop would leave packets going round it indefinitely and gradually choke the network.

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.